[Xen-devel] [PATCH 2/6] x86/boot: Map the trampoline as read-only

[Andrew Cooper <andrew.cooper3@citrix.com>]

c/s ec92fcd1d08, which caused the trampoline GDT Access bits to be set,
removed the final writes which occurred between enabling paging and switching
to the high mappings.  There don't plausibly need to be any memory writes in
few instructions is takes to perform this transition.

As a consequence, we can remove the RWX mapping of the trampoline.  It is RX
via its identity mapping below 1M, and RW via the directmap.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Wei Liu <wl@xen.org>
CC: Roger Pau Monné <roger.pau@citrix.com>

This probably wants backporting, alongside ec92fcd1d08 if it hasn't yet.
---
 xen/arch/x86/x86_64/mm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/x86/x86_64/mm.c b/xen/arch/x86/x86_64/mm.c
index 8ea09ecc30..b7ce833ffc 100644
--- a/xen/arch/x86/x86_64/mm.c
+++ b/xen/arch/x86/x86_64/mm.c
@@ -699,7 +699,7 @@ void __init zap_low_mappings(void)
     /* Replace with mapping of the boot trampoline only. */
     map_pages_to_xen(trampoline_phys, maddr_to_mfn(trampoline_phys),
                      PFN_UP(trampoline_end - trampoline_start),
-                     __PAGE_HYPERVISOR);
+                     __PAGE_HYPERVISOR_RX);
 }
 
 int setup_compat_arg_xlat(struct vcpu *v)
-- 
2.11.0


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